The present invention relates to image sensors, and more particularly, towards a structure for improving the formation of micro-lenses in an image sensor.
Image sensors are electronic integrated circuits that can be used to produce still or video images. Solid state image sensors can be either of the charge coupled device (CCD) type or the complimentary metal oxide semiconductor (CMOS) type. In either type of image sensor, a light gathering pixel is formed in a substrate and arranged in a two-dimensional array. Modern image sensors typically contain millions of pixels to provide a high resolution image. Important parts of the image sensor are the color filters and micro-lens structures formed atop of the pixels. The color filters, as the name implies, are operative, in conjunction with signal processing, to provide a color image. The micro-lenses serve to focus the incident light onto the pixels, and thus to improve the fill factor of each pixel.
Conventionally, micro-lenses are formed by spin coating a layer of micro-lens material onto a planarized layer. The micro-lens material is then developed to form cylindrical or other shaped regions that are centered above each pixel. Then, the micro-lens material is heated and reflowed to form a hemispherical micro-lens. FIG. 1 shows a prior art cross-sectional simplified diagram of an image sensor 101 having micro-lenses formed thereon. As seen in FIG. 1, the image sensor includes a plurality of pixels that have light detecting elements 103 formed in the substrate. The light detecting elements 103 may be one of several types, such as a photodiode, a photogate, or other solid state light sensitive element. Formed atop of each pixel is a micro-lens 105. The micro-lens 105 focuses incident light onto the light detecting elements 103. Moreover, in the region between the light detecting elements 103 and the micro-lens 105, denoted by reference numeral 107, there are various intervening layers that would typically include the color filter layers and various metal conducting lines.
In the prior art, the color filters are formed by the repetitive deposition and etching of the various color filter layers. Typically, there are three color filter layers that are deposited and etched: red, blue and green layers. Alternatively, cyan, yellow, and magenta layers are used. The process of depositing and etching the color filters will sometimes result in an uneven surface. In other words, the color filter layer is not very planar.
This may cause difficulties in forming consistent high quality micro-lenses across all of the multiple image sensor dies on a single wafer. This is because the micro-lens material is spin coated onto the color filter layers. Due to the non-planar nature of the color filter layers, and other irregularities, it has been found that the spin coated micro-lens material is often not evenly distributed across the entire semiconductor wafer.
For example, if a 300 millimeter wafer is used as the substrate to carry the image sensor die, there may be thousands or even tens of thousands of image sensor dies on the single wafer. It is important to be able to evenly distribute the micro-lens material over each of the image sensor dies in a uniform manner.